Uninterruptible Power Supply System UPS arrangements
For providing unbroken power to critical loads during power failures and for continuously regulating the quality of the power, a reliable back up power source of AC power is required. Uninterruptible power supply systems meet this requirement
Components of UPS:
UPS is a static system with equipment like rectifier-charger, battery, inverter and transfer mechanisms. The rectifier charger converts utility AC power to regulate DC power which maintains the battery at full charge and for operations of the static inverter. When the line voltage drops or supply falls, the battery power source supplies energy for the inverter
The transfer system consists of by-pass circuit and a static switch. The by-pass circuit can isolate the UPS from both the power source and the critical load to transfer the load from the preferred power source to an alternative source as required
Different types of UPS:
Different types of UPS systems available are:
- Online UPS
- Offline UPS
- Line Interactive UPS
In Online UPS system ac main power line rectified to DC power and provided power supply to battery bank to charge it and to inverter circuit to convert back to ac to supply power to critical loads.
Inverter will be in continuous service in online UPS. As inverter will be always working in normal operation it is also called as inverter preferred UPS.
If power goes out the inverter and load continue to work on the battery. In Online UPS there will be no switching time during power failure. UPS switches to battery immediatly when main power fails. The loads keep running without any power failure
Static switch is employed to bypass the UPS circuit during the maintenance of rectifiers, batteries and inverters
OFF Line UPS:
Offline UPS is called as standby UPS. In this type of UPS the primary power source is the main power and the secondary power source is the battery
In Offline UPS battery and the inverter does not supply power to the load during normal condition. Battery charger will be charging the battery from main power line during main power available. Battery and the inverter are waiting in the standby mode till they are needed. Hence this type of UPS is called Standby UPS. As main line is primary preferred source it is also called line preferred UPS.
The filters and spike protectors are employed to filter the line noise and surges and to protect the load from severe main conditions
When the ac main power goes out transfer switch detects and automatically switches so that battery stats delivering power to the critical load though the inverter. Battery discharges power during this period and delivers power
In Off line UPS require some finite time to transfer the switch from main power lines to battery inverter path. This time is called transfer time or switching time. This duration will be in the order of few micro sec to milli sec. In Off line UPS system critical loads will not be supplied power during this duration.
Online UPS and Offline UPS difference:
The components in both Offline UPS and Online UPS are same they differ in the design. In Offline UPS inverter will be in standby mode and waiting to take over during the power failure, in Online UPS inverters are continuously running. This eliminate a time gap of switching time or transfer time in online UPS compared to Offline UPS.
The choice of selecting the UPS depends on the criticality of the load for small interruption of switching time
Line Interative UPS:
In this type of UPS separate battery charger, inverter and noise spike filtering unit is replaced with inverter/converter unit. The unit charges the battery during normal condition and converts dc to ac output during main power fails
Under normal operation the transfer switch is closed and ac main power is given to the load through inverter/converter circuit. The converter part of it converts from ac to dc and charges the battery during normal operation
When ac mains fails it operates in reverse order. Transfer switch gets open and inverter/converter circuit acts as inverter. It converts dc power to ac and deliver the power to the load
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